With progress in development of multimedia networks, a demand for dealing with communication traffic is increasing. Under such circumstance, a wavelength division multiplexing (WDM) transmission system configured to relay an optical signal via multiple points and amplifies the optical signal using optical amplifiers plays a major role for achieving economical communication systems in the multimedia society.
For example, in metro core networks where reduction in cost and size is emphasized, WDM transmission systems have been positively introduced. Along with such tendency, a large number of optical transmission apparatuses called optical add-drop multiplexer (OADM) have been introduced to stations.
The number of wavelengths in an optical amplifier for amplifying WDM signals varies arbitrarily from 1 to 40. If only one wavelength is transmitted, only amplified spontaneous emission (ASE) is transmitted through ports where no signal rises as represented in FIG. 3. In this case, after multiple-span transmission, the ratio of signal to ASE (S/ASE) extremely lowers, which makes it difficult to determine whether there is a signal using a photo diode (PD).
To deal with such inconvenience, there is a known technology in which the ASE powers of channels to which no signal has been transmitted are reduced to achieve the spectrum represented in FIG. 4 (see, for example, Japanese Patent Application Laid-open No. 2003-163641).
If such VOA control as represented by FIG. 4 is performed, however, the polarization degree extremely increases compared with the case represented in FIG. 3 in which unpolarized ASE is transmitted. This sometimes causes a physical phenomenon called polarization hole-burning (PHB) in an erbium doped fiber (EDF) of an optical amplifier.
When a high-intensity signal light is input to the EDF, the optical gain of the polarized light parallel to the signal light lowers. This phenomenon is PHB (see, for example, “Erbium-doped Light” published by Optronics Co., Ltd., pp. 59 to 61). The gain of the signal light and the gain of the polarized. ASE parallel to the signal light are reduced due to PHB, whereas a gain of ASE perpendicular to the signal light is not reduced because it is not influenced by PHB. As a result, the ASE perpendicular to the signal light is increased compared with the case where the PHB does not occur.